ABSTRACT Ras proteins are GTPases that activate multiple signaling cascades, which modulate cell survival, proliferation, migration, and differentiation. Ras proteins are regulated by almost all known post-translational modifications, including farnesylation, palmitoylation, phosphorylation, ubiquitination, and acetylation. However, it remains unknown whether Ras proteins are modified by sumoylation. We recently discovered that all three forms of Ras protein are SUMO3-modified. Our further studies reveal that lysine 42 (K42) is the primary residue for sumoylation, whereas PIAS? and MEKK1 are candidate E3 ligases that sumoylate Ras. Sumoylation is essential for activating K-Ras and its downstream signaling. Furthermore, we have discovered a series of SUMO E1 inhibitors that target SUMO conjugation both in vitro and in vivo. The lead compounds 27A and SB-264 potently inhibit sumoylation, but not ubiquitination or neddylation. Significantly, these compounds display excellent bioactivities toward killing transformed pancreatic cells with K-Ras mutations at glycine 12 or glycine 13 and suppress their migration. Given the role of sumoylation in controlling functions of key cellular regulators and the paramount importance of K- Ras in cell proliferation and transformation, we hypothesize that sumoylation promotes K-Ras activity and that the SUMO regulatory pathway is an excellent target for development of anti- cancer drugs. Three Specific Aims are proposed to test the validity of the hypothesis. We will determine the mechanism by which sumoylation regulates K-Ras activation, elucidate the regulatory network that controls K-Ras sumoylation, and investigate the efficacy of SUMO E1 inhibitors in suppressing proliferation of pancreatic cancer growth in preclinical mouse models. Further characterization of K-Ras sumoylation promises to have a profound impact on our understanding of the fundamental cellular signaling mechanisms that regulate cell growth. Given the discovery of novel SUMO E1 inhibitors, our proposed studies will help to accelerate the development of therapeutic applications that target ubiquitin-like modifications. !